Cast-iron is an iron-carbon alloy, well-known and widely used for the manufacture of mechanical parts. Cast-iron is obtained by mixing the constituents of the alloy in the liquid state at a temperature comprised between 1135° C. and 1350° C. before casting in a mold and cooling of the obtained alloy.
During its cooling, carbon may adopt different physico-chemical structures depending on several parameters.
When carbon is associated with iron and forms iron carbide Fe3C (also called cementite), the resulting cast-iron is called white cast-iron. White cast-iron is characterized in that it is hard and brittle, which is not desirable for some applications.
If carbon appears in the form of graphite, the resulting cast-iron is called gray cast-iron. Gray cast-iron is softer and may be worked.
In order to obtain cast-iron parts having good mechanical properties, it is therefore necessary to obtain a cast-iron structure comprising as much as possible carbon in the form of graphite and limit as much as possible the formation of these iron carbides which harden and embrittle the alloy.
In the absence of any particular treatment, carbon tends, nonetheless, to be associated with iron so as to form iron carbide.
Hence, it is necessary to treat the cast-iron in the liquid state so as to modify the association parameters of carbon and obtain the desired structure.
To this end, the liquid cast-iron undergoes an inoculation treatment aiming to introduce, in the cast-iron, graphitizing compounds which will promote, when the cast-iron is cooling in the mold, the apparition of graphite rather than iron carbide.
In general, the compounds of an inoculant are elements which promote the formation of graphite during the solidification of the cast-iron. For example carbon, silicon, calcium, aluminum, etc.
Of course, an inoculant may also be designed so as to fulfill other functions and, to this end, comprise other compounds having a particular effect.
In particular, depending on the required properties, it may be desired that the formed graphite is spheroidal, vermicular or lamellar. Either one of the graphitic forms can be obtained preferably by a particular treatment of the cast-iron by means of specific compounds. Thus, for example, the formation of spheroidal graphite may be promoted by a treatment called nodularizer treatment mainly aiming to provide the cast-iron with enough amount of magnesium so that graphite may grow so as to form rounded particles (spheroids).
For example, these nodularizer compounds may be included in the inoculant alloy.
We can also mention the addition of desulfurizing products, or products that allow treating specifically some defects of the cast-iron depending on the initial composition of the liquid cast-iron bath, such as micro shrinkage cavities, likely to appear during cooling. In particular, it may consist of lanthanum and rare-earth elements.
These treatments may be performed at once or in several times and at different moments during the manufacture of the cast-iron. In particular, it is known to add the inoculant in the ladle, prior to the casting of the cast-iron in the mold (ladle inoculation), during casting, or still in the casting jet (late inoculation).
Most inoculants are conventionally manufactured from a FeSi65- or FeSi75-type ferrosilicon alloy with the adjustment of the chemistry according to the aimed composition of the inoculant. The adjustment is possible in furnace or in ladle, with usually poor efficiencies, depending on the elements to be added. It may also consist of mixtures of several alloys.
It should be noted that the inoculation effectiveness of the cast-iron part also depends on its thickness.
In areas with small thicknesses, which cool more quickly, a higher risk of carbides formation will be noted.
Conversely, in areas with larger thicknesses, cooling will be slower and will promote the formation of graphite. Nonetheless, in parts with large thicknesses, cooling may be too slow and the formed graphite may lose its nodularity in the vicinity of the center of the part.
As a result, parts having areas with different thicknesses can have different physico-chemical structures from one area to another, which is not desirable.